Date   : Mon, 09 Feb 2004 16:30:11 -0000
From   : "Richard Gellman" <splodge@...>
Subject: Re: Discs

BBC Floppies, in full:

The original DFS discs are Single Sided, Single Density (SD) discs. They
hold 100K or 200K per disc surface. A 40-track disc holds 100K, an 80-track
disc holds 200K.

Now, single sided does not mean "limited to one side". It just means that
DFS only reads one surface as a "disc". If you have a double-sided drive,
you can read the other side with *DRIVE 2. (Note that turning the disc over
does not work).

ADFS Discs come in 3 flavours: 160K ("S" size, 40-track, single-sided,
double density (DD)), 320K ("M" Size, 80-track, single-sided, double
density), and 640K ("L" size, 80-track, double-sided, double density).

High Density (HD) discs won't work with BBC Micros. That is, any 5.25"
floppy can be reformatted to single or double density, but if they are
formatted to High Density (known as the 1.2Mb (1.44Mb for 3.5" discs)
floppies in the PC World) then they simply won't be recognised.

Disc drives themselves come in different flavours too. PC 5.25" floppy
drives generally don't work with BBC discs, although they can be used, they
have a different track stepping, and so any disc you format in them won't
work in proper BBC disc drives.PC 5.25" discs can however be reformatted to
BBC Micro geometry/track stepping.

BBC Disc drives are 40-track, 80-track or 40/80 switchable. Some will claim
there is no such thing as an 80-track only drive, but I have seen them and
can confirm the existence of these bizarre units. 40-track drives cannot
read 80-track drives, but 80-track drives can "double-step" to read 40-track
discs. 40/80 switchables are the ideal drives as they can be configured to
be native for whatever disc format you are using.

Not all disc drives are double-sided either. Some of the earlier cheaper
units were sold with reading single-sided DFS discs in mind. and so only
have one head. They cannot be used with ADFS L discs, but DFS, and ADFS S/M
discs will work in these drives (80-track drive needed for "M" discs).

There are also some propietary disc formats around as a result of
third-party DFSes. One in particular is the Watford DDFS, which uses a DFS
disc structure, but in Double Density mode. I have no idea what the exact
capacity of this format is, but it is larger than the average 100K/200K DFS
disc.

Hope this helps you.

-- Richard Gellman

----- Original Message -----
From: "Colin" <cwhill@...>
To: <bbc-micro@...>
Sent: Monday, February 09, 2004 3:52 PM
Subject: [BBC-Micro] Discs


> Ok, now that I actually have a BBC Micro with a disc drive, could someone
> point me in the right direction for finding info on the discs themselves
> (what the specs were, ssd's etc) and also if they are still available for
> sale. I have looked at various sites that seem to sell the right size
discs
> but all those letters are confusing me.
> Sorry to have to ask such a basic question but we all have to start
> somewhere (all my stuff was on cassette tapes).
> Thanks,
> Colin Hill
>
>
>